GB2102482A - Offshore mooring structure - Google Patents

Description

1 GB 2 102 482 A 1

SPECIFICATION

An offshore mooring construction This invention relates to an offshore mooring construction, to which a surface vessel may be moored, especially for loading and unloading by connection to a submerged pipeline laid on a deep sea bed. The construction of the present invention is of particular application in the exploitation of oilfields situated offshore on very deep sea beds, but the construction according to the present invention can be used with advantage also for other purposes.

For mooring purposes, it is known to provide systems which are mainly based on buoy systems connected 10 to the sea bed by chains, with tubular legs or latticework legs with articulations such as to have the connections to the sea bed working essentially under tensile stresses.

When a horizontal pulling stress is imparted to such a mooring system, the buoy is caused to be displaced so that it becomes more deeply immersed. When the horizontal pulling stress is discontinued, the buoy tends to be brought back to its original position by the increased buoyancy which has been caused by the 15 deeper immersion. In this connection, the following prior art disclosure can be cited, namely French Patents Nos. 2 137 117, 2 159 703,2 187 596, 2 200 147,2 307 949, 2 367 654,2 375 087 and 2 386 758, and United States Patents Nos. 3 407 416,3 614 869, and 3 899 990.

However, such prior art systems create serious problems associated with connecting the pipeline, which conveys the fluid from the sea bed to the surface, in view of the required articulation, especially if the sea bed 20 is at a great depth.

Such connection can be embodied by a hose which, however, undergoes considerable stresses, both owing to the fatigue induced by repeated bendings and owing to the squeezing pressure when the hose is empty, the latter pressure being capable of becoming prohibitive with very deep sea beds.

Another possible mode of connection is that using articulated joints. On the articulated joint approach there are numerous patents such as French Patents Nos. 2 367 000,2 377 546,2 348 428 and 2 406 746 and British Patent No. 1 549 756.

The adoption of articulated joints at great depths creates a number of problems both owing to the variety and magnitude of the stresses which the joints are supposed to withstand and owing to their positioning and upkeep.

The connection joints adopted most frequently are of the spherical or the Cardan type as they are required to be rotated in all directions. The tightness of the seal of such joints is a source of many problems.

The types of connection which are most heavily stressed should always be fitted with a barrier valve, for isolating the joint to permit work to be carried out on the joint. Such a valve, which is very bulky and must be automatically controllable, is a further source of complications and of cost increase.

For these reasons, especially when the mooring construction is to be installed at great depths, that is over metres, the constructions provided by the known art have a number of defects both as to the operations necessary for their erection and as to their practical use.

The most serious difficulties are experienced in the hinge which secures the construction to the sea bed and the connection of the pipeline laid on the sea bed to the pipeline which comes down from the surface. 40 Such movable component parts are subjected to considerable stresses and their upkeep, or replacement if necessary, involves very high costs both from the point of view of operation and in terms of lost output.

In this respect, it is to be appreciated that, when a mooring facility is not available for crude oil loading, the exploitation of the offshore oil field concerned may need to be discontinued and the tanker ships which cannot load remain unused.

In the case in which the pipeline from the sea bed to the surface is required to convey, ratherthan a liquid phase, a slurry of solids in suspension, the problems involved with thejoints become even more serious.

The tanker ship is usually secured by its bowto the mooring structure by one or more hawsers. The ship can rotate about the mooring point so as to minimize the stresses caused by wind thrust, sea currents and waves impinging thereon, thereby minimising the stress on the entire mooring construction.

According to the present invention, there is provided a construction suitable for use in mooring a ship offshore, which comprises a broad rigid foundation member and, extending upwardly therefrom, a slender vertical or substantially vertical structure having a flexural resistance modulus which decreases from the foundation member in the upward direction.

For a better understanding of the present invention and to show how the same may be carried into effect, 55 reference will be made, over much of the remainder of the description, by way of example, to the accompanying drawings, in which:

Figure 1 is a perspective view of one embodiment of mooring construction in accordance with the present invention:

Figure2 is a perspective view of a different embodiment of mooring construction in accordance with the 60 present invention; Figure 3 is a side view of the embodiment of Figure 1; Figure 4 shows, on an enlarged scale, a vertical section through an upper portion of the embodiment of Figure 1; Figure 5 shows, as an elevation, stages 1 to X in the erection of an embodiment of mooring construction as 65 2 GB 2 102 482 A 2 shown in Figure 11; and Figure 6 shows, as an elevation, a system for ballasting the embodiment of Figure 1.

The structure of the construction may comprise for instance, a solid walled cylindrical tower (Figure 1) having a variable cross-section as shown in Figure 1, or a latticework structure as shown in Figure 2, or a combination of the two structural types. Such a vertical structure is rigidly connected to a broadened base foundation block placed on the sea bottom and stably positioned thereon by its own weight and/or by its being secured to the sea bed by foundation poles driven thereinto.

The slender vertical structure can be made of, for instance, steel or reinforced concrete, or a combination of such two materials.

In addition, it can be stabilized by an inert material which can be introduced therein before, or also after, 10 the launching of the construction, using specially provided hollow spaces therein.

In practice, the vertical structure emerges above the sea and supports at its top end region a rotary table to which equipment required for mooring and ship loading may be secured. Such equipment can comprise, for example, in addition to the rotary table which permits the structure to be oriented along the direction of the hawser pull when mooring the ship, a rotary joint in order to permit the flow of fluid irrespective of the orientation of the superstructure, and a loading boom to support, above the bow of the moored ship, the loading hoses connected to the rotary joint.

Moreover, the superstructure may accommodate other installations such as machines for pumping and metering the flow of fluid, safety and communication apparatus, emergency dwellings for the attendants charged with upkeep and operation, and a helicopter landing area for the transportation of personnel to and 20 from the structure. The nature and use of such installations are conventional.

According to a preferred embodiment as depicted in Figure 3, the slender vertical structure has a buoyancy chamber secured thereto and preferably at a depth, p, as defined by the formula p = K11--- where K, varies from 0.12 to 0.30, the preferred range being from 0.15 to 0.20, L is the overall height of the mooring construction, and the distance p is measured from the top towards the bottom.

Such a buoyancy chamber affords considerable advantages. A first advantage is to produce, as the structure undergoes a lateral pull, a counteracting moment which tends to restore the structure to its vertical position. In addition to that, the surface of the buoyancy chamber acts like a hydrodynamic dampening member to counteract any swinging motions of the structure.

The buoyancy thrust, furthermore, has a considerable attenuating effect towards the combined bending 35 and compressing stresses which are considerable in a slender structure.

In a preferred embodiment now considered, the variation of the resisting cross-sections along the axis of the structure, in the portion between the buoyancy chamber and the point of connection of the mooring structure to the foundation base, is in accordance with the formula:

40 K.1.

J70 45 wherein, with reference to Figure 3, J is the flexural moment of inertia of the section concerned, at a distance x from the buoyancy chamber, JO is the flexural moment of inertia in the section in the region of the connection of the structure to the buoyancy chamber, LO is the distance between the buoyancy chamber and the point at which the structure is connected to the foundation base, and K2 is a numerical coefficient (no 50 dimensions) in the range from 1.6 to 2.5 and prefrably in the range from 1.9 to 2.1.

Such a law of variation, as expressed by the formula reported above, permits materials to be exploited according to constant coefficients and prevents waste or oversizing of component parts.

In practice, the slender structure as shown in Figure 1 is built as discrete portions each having a constant cross-sectional dimension. The trend of the flexural moments of inertia, and thus of the resistance moduli 55 (flexural, along the vertical axis of the mooring structure is thus that of a broken line in agreementwith the formula reported above.

In the case in which the slender structure is composed of tubular structural members, the variation of the flexural moment of inertia can be obtained by building the several discrete portions with different diameters and/or wall thicknesses.

In the case in which the structure is built according to a latticework pattern, the characteristics of stiffness of the lattice sections will be varied by changing the design and,'or the cross-sectional areas of the individual truss components.

A characteristic feature of the mooring construction according to the present invention is that the emerging slender structure, which, together with the foundation base and the mooring hawsers, makes up 65 3 GB 2 102 482 A 3 the basic element for securing theta nker ship to the sea bed and which is also a structure for supporting the equipment required for the mooring and loading operations, is rigidly fastened to the foundation and provides, by virtue of the distribution of the moments of inertia therealong, a static and dynamic behaviour which is extremely advantageous.

Such behaviour is radically different from those of the conventional art as discussed hereinabove. 5 The construction according to the present invention has a static behaviour corresponding to a resilient rebound characteristic for the structure, as a function of the mooring stress, typically in the range from 6 to metric tons per metre of displacement (at the level of the mooring location proper), consistent with the environmental conditions and the size of the ship concerned.

Such a structural "yieldability" has proven to be very useful both to limit the pulling loads in the mooring 10 hawsers when the ship is moored (and thus exposed to the thrusts of the waves and the wind) and thus pulls and releases the hawsers, and to limit the localized bumping stresses in the case of an accidental collision of the ship when approaching the mooring position for placing the mooring hawsers and loading hoses in position.

The dynamic behaviour of the structure, especially for use on very deep sea beds such as those over 300 metres, can be definitely peculiar. As a matter of fact, the structure has a first natural swinging mode, shown in Figure 3 at A, which has a period longer than 35 seconds, that is a period longer than the maximum period length as is known from oceanographic observations.

The structure also has a second swinging mode, which is indicated at B in Figure 3, which has, along with the swinging modes of higher order, a. period of its own which is shorter than 7 seconds, that is shorter than 20 the period of possible waves of small period but with a significant impact strength.

The characteristic slender outline of the structure forming part of the construction according to the present invention ensures that, for the first swinging mode, the structure has both the appropriate resistance in the static behaviour, and a low dynamic amplification factor for all of the swinging modes.

This is attributable to the fact that the possible proper swinging periods are different in a sharp manner 25 from the field of the periods of the possible waves having a high impact strength.

Thus, the occurrence of considerable resonance phenomena is prevented and, consequently, the occurrence of fatigue stresses at the points in which such stresses concentrate.

In order that such a behaviour relative to the dynamic stresses may fully be appreciated, it is fitting to consider that the slender structure forming part of the construction according to the present invention undergoes stresses having a cyclical nature as caused by the environmental conditions, such as the wave motions, the pull of the mooring hawsers and the wind thrust.

An elastic structure subjected to pulsatory stresses can vibrate according to a very great number of swinging modes, which are identified by the circumstance that the lines of maximum elastic deformation have an increasing number of "nodes", that is, of points of intersection with the vertical line which is the 35 configuration in the undisturbed condition.

in Figure 3 there are indicated the first two swinging modes which are the most significantfrom the point of view of the energetic magnitude of the stresses.

In the geographical areas of greatest interest the distribution of the wave periods for waves having the most significant power contents varies between 6 and 20 seconds. To prevent phenomena of dynamic reinforcement of the oscillation of the structure it is necessary that the natural period of oscillation of the structure, according to any of the possible modes of oscillation thereof, is the farthest possible from the periodsproper of the impinging waves.

To prevent resonance phenomena of the kind referred to above, the offshore structrues of the conventional art have periods proper of vibration which are reasonably lower than the periods of the significant forces originated by the waves. Such structures have maximum clisplacements close to the conditions of static load relative to the magnitude of the wave forces at every instant of time.

This requirement involves a much stiffer structure as well as the use of a greater amount of building materials.

With the structure according to the present invention, conversely, the result is that, for the first oscillation 50 mode - reported as A in Figure 3, and in the case of actual practical interest in deep water (250m-500m of depth)- the structure as such has a proper period of swinging which is considerably iongerthan that of the longest waves, that is the waves the period of which is the longest. Under such conditions, the structure behaves like a flexible or yieldable structure that is a structure which is capable of accompanying with its elastic deformations the variable field of wave forces, thereby reducing the magnitude of the hydrodynamic 55 forces which are actually transferred to the structure.

For the second swinging mode, indicated as B in Figure 3 and still more intensely for the swinging modes of the high orders, the result is that the natural period is shorter than that of the small-period waves but the energetic content is still considerable so that such waves can stress the structure in a significant way.

0 This fact takes place principally in the field of the waves and thus of the loads which are capable of impressing fatigue stresses to the structure, because of the high number of probabilities of having to deal with waves having such characteristics.

The particular position of the buoyancy chamber is such as to produce the effect of increasing the period proper relative to the swinging mode A because such a mode favourably influences the inertial characteristics of the elastic system as represented by the structure. Conversely, for the second swinging 65 4 GB 2 102 482 A 4 mode, inasmuch as the buoyancy chamber is located near a node of the maximum elastic deformation line, the chamber does not influence the features of the system considerably so that the swinging period relative to that mode is virtually unaffected.

In Figure 1, the slender emerging structure 1 is in the form of a tubular column which is rigidly secured at its lower region to a foundation base 2 composed of a]attic work made of tubular members. A region 3 of the structure 1 is where there is a rigid insertion connection relative to the foundation base and it will have the greatest stiffness relative to the regions of the structure, as considered by proceeding from bottom to top.

The foundation base 2 rests on the sea bed by means of foundation blocks 4 (of which there are three in the embodiment shown in the drawing).

The weight of the structure, complete with ballast, is sufficient to counteract with the end reactions the normal forces and the upturning moments due to the weight of the structure, to the external causes in action and the environmental conditions, such as wind force, currents, waves, or the working conditions such as the pull of the mooring hawsers, accidental overloads and others.

In addition to, or as an alternative to, the exploitation of their own weight, the blocks 4 can be secured to poles driven into the sea bed by hammering with an underwater hammer and subsequent cement injection. 15 To the top end region of the emerging slender structure 1, a rotary table 5 is secured and, on it, there is supported a superstructure 6 with the attendant diagrammatically symbolized equipment and machinery, viz. a mooring hawser 7, a loading boom 8, a hose 9 for transferring crude oil to a moored tanker ship 10, and a helicopter landing area 11.

One or more pipeline 12, housed in the vertical structure, connects the sea bed to the sea surface and is 20 connected to the pipeline 13 laid on the sea bed. The connection system for the two pipelines 12 and 13 can be made by a welded joint placed within a sealtight compartment 14 which can be maintained under atmospheric pressure and to which an operator may have access by a caisson-like bell.

Figure 2 illustrates the case in which the emerging slender structure 1 is embodied by an open-meshed latticework structure or truss.

In Figure 2 the same reference numerals have been adopted as for Figure 1, and the same considerations apply.

Figure 4 is a diagrammatic representation of an upper portion of the mooring construction of Figure 1. Here, the top end portion of the structure 1 is connected to the superstructure 6 by the rotary table 5, or o bearing, which permits rotation about the vertical axis. The vertical pipeline 12 for conveying the product can 30 communicate with a device 16 for holding and inserting so-called "pigs" for cleaning the interior of that pipeline and for displacement of a fluid in that pipeline, the device 16 having an accessing valve 17 and a high-pressure pneumatic circuit 18. The pipeline 12 is also in communication, via a cutoff valve 19, with a rotary hydraulic joint 20, placed on 35 the rotation axis of superstructure 6, for connection to a duct 21 supported by the loading boom 8 and to the 35 hose 8. The hose 9, in its turn, is connected, during the loading operations, to a pipeline 22 for loading the tanker ship 10, by means of a quick-lock joint 23. The mooring hawser 7 connects the superstructure 6 to the tanker ship 10. 40 When no loading operation is in progress, the hose 9 is allowed to hang vertically with its free lower end 40 connected to a rope 24 which can be used to haul the hose aboard the vessel 10. From the foregoing description it is apparent that a significant advantage of the construction according to the present invention lies in its submerged portion being completely monolithic and, as such, it does not require any sophisticated construction or special hydraulic and mechanical upkeep operations for the submerged portions; this was a critical point of the conventional structures as used hitherto.

The construction according to the present invention can be manufactured both simply and cheaply: in the following an erection procedure with will be described along with the constructional procedure, by way of example only and without limitations: from this description the ease and the simplicity of the construction will become clear.

With reference now to Figure 5, the constructional and erection stages are the following. In stage 1 the vertical structure 1 and its foundation base 2 and blocks 4 are fabricated in discrete sections having the appropriate length, in a shipyard. In stage 11 such sections are launched separately and structurally connected when afloat, the operation being carried out in a confined water enclosure.

In stage Ill the construction is then connected at a number of points to auxiliary buoyancy means by cables or chains and is loaded, for example by flooding it partially by appropriate flooding valves, until a stable 55 horizontal submerged position is attained.

In such a position the construction is towed (in stage IV) to the installation site; the shipment in submerged position minimizes the dynamic bend action and thus stresses to the structure.

Once the erection site has been reached, the construction is restored to its floating condition (in stage V) by dumping the added weight, for example by displacing the ballast water which has been introduced during 60 stage Ill, by compressed air fed by hoses from the depot barge, whereafterthe auxiliary buoyancy means are disconnected from the structure.

In stage V1 a few compartments of the construction are gradually flooded so as to have it capsized until a stable vertical floating posture is attained. An additional introduction of ballast water, in stage Vil, permits the construction to come to rest on the sea bed.

?5 GB 2 102 482 A 5 If the solution exploiting the weight is adopted, solid ballast is introduced, in stage V111, into the foundation blocks 4 to achieve the static stabilization of the entire construction; as an alternative, the blocks 4 may contain beforehand the necessary ballast quantity to make sure that, once the installation has been completed, there is stability on the sea bed. In such a case the foundation blocks 4 have buoyancy chambers which enable the blocks to be shipped afloat, to be flooded subsequently during the laying operations.

The stability on the sea bed can also be achieved by securing the foundation block or bases to poles driven into the sea bed and then cemented in position.

During the subsequent stages, there are mounted (in stage IX) the intermediate structures by a crane mounted on a pontoon and (stage X) the connection with the sea bed pipeline is made by using a caisson 10 type machine.

In Figure 6 there is shown, byway of example without limitation, a diagram of the ballast system which may be used forthe operation described above, both for shipping and for erection, according to which water is introduced first as a ballast, and solids forthe same purpose thereafter.

For practical reasons, the solid ballast material is preferably slurried in water in a divided form such as 15 granules of a discrete dimension, pebbles, or large grit dust. The water used for the conveyance is then caused to escape through escape valves.

A hose 25 is connected by a quick-lock joint 26 to a distribution system 27. From this system it is possible by valves 28 controlled from a remote location, to send liquid or solid ballast material to the intended ballast compartment placed in the structure or base or blocks, by actuating a pump 29 and opening a valve 30, both w installed aboard the tanker.

Valves 31 permit the venting of air and/or the discharge of the conveyance fluid in the case of an aqueous slurry of a solid ballast material.

Claims (13)

1. A construction suitable for use in mooring a ship offshore, which comprises abroad rigid foundation member and, extending upwardly therefrom, a slender vertical or substantially vertical structure having a flexural resistance modulus which decreases from the foundation member in the upward direction.

2. A construction according to Claim 1, wherein to an upper region of the slender structure there is rigidly secured a hollow buoyancy body which is completely immersed.

3. A construction according to Claim 2, wherein the buoyancy centre of the hollow body is located at a depth in the range from 12% to 30% of the depth at which the slender structure is connected to the foundation member.

4. A construction according to Claim 3, wherein the buoyancy centre of the hollow body is located at a depth in the range from 15% to 20% of the depth at which the slender structure is connected to the foundation member.

5. A construction according to Claim 2,3 or4, wherein the flexural moment of inertia of the slender structure is increased in the portion between the buoyancy body and the point of connection with the foundation member according to the formula:

7, 1 K J0 J.

wherein J is the flexural moment of inertia of a section situated at a distance xfrom the buoyancy body, JO is the moment of inertia of the section having the connection with the buoyancy body, LO is the length of the 45 portion between the buoyancy body and the point of connection with the foundation member, and K2 is a coefficient (non-dimensional) in the range from 1.6 to 2.5.

6. A construction according to Claim 5, wherein K2 is in the range from 1. 9 to 2.1.

7. A construction according to any preceding claim, wherein the slender structure is composed of a jo hollow tubular structure having a cross-sectional area variable in the lengthwise direction.

8. A construction according to anyone of Claims 1 to 6, wherein the slender structure comprises a tridimensional latticework structure.

9. A construction according to anyone of Claims 1 to 6, wherein, in the slender structure, cylindrical component parts are associated with latticework component parts.

10. A construction according to any preceding claim, wherein the slender structure is made of steel 55 and/or reinforced concrete.

11. A construction according to any preceding claim, wherein the foundation member is laid on the sea bed and secured thereto by the presence of solid ballast material in a hollow compartment of the foundation member and/or of the slender structure, such ballast material having the form of comminuted bodies.

0

12. A construction according to Claim 11, wherein associated with the or each hollow compartment is a 60 means for ejecting fluid from that compartment.

13. A construction according to any preceding claim, wherein the foundation member is secured to the sea bed by poles driven into the sea bed and united to the foundation member by cement injections.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1983. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.